A nose cone assembly is typically found at the intake of a gas turbine engine. The assembly conventionally comprises a one or two piece nose cone mounted on a support ring that is in turn fixed to the fan disk. The purpose of the nose cone assembly is to provide smooth inflow of air to the fan blades and to protect the rest of the engine from damage that could be caused by a foreign body impact such as a bird strike.
A typical two piece nose cone assembly 2 is illustrated in FIG. 1. The nose cone 4 comprises a woven glass fibre fabric pre-impregnated with an epoxy based resin and is manufactured using a hand lay-up technique. The thickness of the nose cone 4 and the cone angle are determined by the required impact behaviour. The thickness of the nose cone 4 is determined empirically based on impact test experience and increases with radius. The angle of the nose cone 4 is determined according to both impact behaviour and the necessity to avoid ice build up on the surface of the nose cone 4. The outer surfaces 6 of the nose cone 4 are coated with polyurethane for erosion resistance on top of an epoxy based paint. A white spiral is painted onto the nose cone 4. Typically, the nose cone 4 is radially located using a spigot fit 8 onto an aluminium support ring 10 and is axially located using a bolted flange 12 that mates with a corresponding flange 14 on the support ring 10. The nose cone 4 is a rotationally balanced component and is circumferentially timed using dowels. Any additional component balancing is achieved using steel putty. The second piece of the nose cone assembly is the fairing 16 that maintains the annulus line between the nose cone 4 and an adjacent annulus filler (not shown). The fairing 16 is typically made from the same composite material as the nose cone 4. Countersunk screws locate the fairing onto to discrete brackets 18 that are attached to the main nose cone support ring flange 14. The rear of the fairing 16 provides a support location for the leading edge of the annulus filler (not shown). The fairing 16 is painted and coated for erosion protection in the same manner as the nose cone 4. A leading edge seal is fixed to the nose cone 4 to prevent fairing vibration.
In the event of a foreign body impact, such as a bird strike, all of the impact forces are carried by the nose cone 4. Deflections into the support ring 10 could cause undesirable strains in the support ring and are thus minimised as far as possible. The connection between the nose cone 4 and the support ring 10 is highly rigid, ensuring any deflection caused in the nose cone 10 is limited to remain forward of the change in cone angle, illustrated at 20 in FIG. 1, and is not passed to the support ring 10.
In order to simplify the manufacturing process, attempts have been made to design one piece nose cone assemblies, such as for example those disclosed and illustrated in US2008/0022524 and U.S. Pat. No. 6,416,280. These one piece nose cones may be attached to the support ring by angled or pocketed screws that engage the support ring flange. However, in order to ensure the necessary rigidity in the connection between nose cone and support ring, such one piece designs require comparatively thick ring sections to impart the necessary stiffness. Particularly in a large nose cone assembly, such increased thickness carries a heavy weight penalty. In addition, manufacturing issues can arise with such thick sections in a composite material, including issues with inadequate wetting, induced thermal and cure shrinkage stresses, internal cracking and delamination. However, if the thickness of the attachment sections were reduced, then high deflections would be experienced within the nose cone and passed to the support ring. Following impact with a large bird, such deflections would be sufficiently high to raise concerns over excessive strains in the support ring that could lead to failure. It is therefore desirable to provide a lightweight nose cone assembly that is nonetheless highly impact resistant.